NITED STATES DEPARTMENT OF AGRICULTURE 

BULLETIN No. 1013 

Contribution from the 
Bureau of Markets and Crop Estimates 

H. C. TAYLOR, Chief 


Washington, D. C. PROFESSIONAL PAPER December 22, 1921 




THE INFLUENCE OF RELATIVE HUMIDITY AND MOIS¬ 
TURE CONTENT OF WHEAT ON MILLING YIELDS AND 
MOISTURE CONTENT OF FLOUR. 

By J. H. Shollenbergee, Grain Supervisor, in Charge Milling Investigations. 


CONTENTS. 

Page. 


Introduction_ 1 

Humidity_ 2 

Normal moisture content_ 2 

Material and method of experiments_ 3 

Influence of relative humidity on total yield of mill products and invisible loss_ 4 

Comparison of the influence of relative humidity and of moisture content of wheat 

on milling yields_ 6 

Influence of relative humidity on moisture content of flour_ 7 

Comparison of the influence of relative humidity and of moisture content of wheat 

on the moisture content of flour_ 8 

Influence of atmospheric temperature ou the total yield of mill products_ 10 

Summary__j_ 10 

t / M 

■ 


INTRODUCTION. 

It is common knowledge among millers that the total weight of 
the products obtained in milling almost invariably differs from the 
weight of the wheat used. This difference in weight is due largely 
to changes occurring in the moisture content of the wheat during 
its preparation for grinding and throughout the process of milling; 
therefore, any factor influencing the amount of moisture which be¬ 
comes added to wheat or evaporated therefrom in its preparation 
for milling and during the different steps of milling influences to 
that extent the percentage yield of mill products. 

The results of certain milling tests conducted in the experimental 
mill operated by the Federal Bureau of Markets and Crop Estimates 
at Washington, D. C., indicate the nature of some of these factors and 
the extent of their influence. Information was obtained on the fol¬ 
lowing subjects: The influence of the relative humidity of the air 
within the mill and of the moisture content of hard wheats when 

68709°—21 



Monograph 























2 BULLETIN 1013, U. S. DEPARTMENT OF AGRICULTURE. 

tempered to 15 per cent moisture on the invisible losses occurring 
during the milling process and on the total yield of mill products; 
also the influence of the temperature of the air within the mill upon 
the total yield of mill products. 

HUMIDITY. 

That the humidity of the atmosphere in the mill influences milling 
results is commonly known to millers. This influence, however, the 
miller is able to control to a certain extent by varying the tempering 
or conditioning process applied to the wheat and the manner of 
grinding and bolting, provided he can anticipate any change that 
is likely to take place in the humidity of the atmosphere. Advance 
information of the weather, however, can not always be depended 
upon; consequently, the miller can not always have his wheat condi¬ 
tioned and his mill adjusted properly for obtaining the highest de¬ 
gree of efficiency possible. Certain atmospheric conditions are gen¬ 
erally recognized as being more conducive to good milling results 
than others ; therefore, the installation of means for controlling the 
condition of the atmosphere within the mill would give to the miller 
a greater degree of control over the operation of his plant and there¬ 
by improve his results. 

NORMAL MOISTURE CONTENT. 

Evidence that an approximate parallelism exists between the mois¬ 
ture content of wheat and the relative humidity 1 of the atmosphere 
lies in the well-known fact that the normal moisture content of air- 
dry wheat is higher when stored in moist climates than when stored 
in dry climates. The term “ normal ” as used in this bulletin means 
that moisture content which is unaffected by continued exposure to 
the prevailing atmospheric conditions. In other words, it is that 
point at which equilibrium is established between the moisture con¬ 
tent of the wheat and the humidity of the air. According to experi¬ 
ments by Stockham, 2 the moisture content normal for different wheats 


1 The amount of water vapor which can be mixed with a given quantity of air varies 
with its temperature; the higher the temperature, the greater the amount of water vapor 
it can hold. When air at a given temperature contains all the moisture possible for it 
to hold, it is said to be saturated. The percentage or ratio of the actual amount of 
moisture contained by a cubic unit of air to the amount which the same air would hold 
at the same temeprature if sa-turated is called the relative humidity. Relative humidity 
is usually stated in terms of percentage. 

Since the capacity of the air for water vapor increases proportionately with tem¬ 
perature, every increase in temperature, assuming that the quantity of water vapor in 
the air remains the same, results in a lowering of the relative humidity, and every 
decrease in temperature results in a proportionate increase of relative humidity until 
the saturation point is reached, beyond which condensation occurs. 

2 Stockham, W. L., The Capacity of Wheat and Mill Products for Moisture. North 
Dakota Agr. Exp. Sta. Bui. No. 120, 1917, 

LIBRARY OF CONGRESS 

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INFLUENCE OF RELATIVE HUMIDITY AND MOISTURE CONTENT. 3 

and for their various mill products varies somewhat, even under the 
same atmospheric condition. 

In milling, the practice of adding moisture to wheat in its prepara¬ 
tion for grinding, which is usually referred to as tempering, may 
give to the wheat a moisture content above normal for the prevailing 
atmospheric conditions. Whenever this occurs and the wheat is ex¬ 
posed to the air, evaporation of this excess moisture begins. The 
quantity of moisture, however, actually evaporated from the wheat 
during milling is dependent upon a number of variables, including 
the quantity of moisture in excess of that normal for the prevailing 
atmospheric conditions, the distribution of the moisture, the length of 
time the material is exposed, the manner of exposure, the circulation 
of air, and the character of the material. Furthermore, because of 
the heat generated by the action of the various machines used in mill¬ 
ing and other causes, the relative humidity of the air in different 
parts of the mill varies somewhat, affecting the rate and extent of 
the evaporation taking place at different steps in the milling process 

MATERIAL AND METHOD OF EXPERIMENTS. 

The wheats used in the experiments conducted by the Bureau of 
Markets and Crop Estimates were milled on a special-type reduction 
machine, consisting of four pairs of 6-inch rolls, three corrugated and 
one smooth, together with a sifter and sieves appropriate for making 
the various separations of stock required. The quantity of wheat 
used in each test was 1,500 grams, or approximately 3 pounds. The 
wheats tested were hard wheats, representing various grades, quali¬ 
ties, and varieties, from various parts of the United States. 

The system of milling used, although not the continuous automatic 
system employed in commercial mills, included all the various steps 
deemed necessary in modern milling practice. Figure 1 represents 
the flow sheet of the system used. 

The time required to make each test averaged 1^ hours, which 
incidentally represents the period of time that each sample was 
exposed to the atmospheric conditions prevailing in the room in 
which the milling was performed. Before milling, each sample was 
cleaned, scoured, and tempered. No cleaning or scouring was per¬ 
formed after tempering. A determination of the moisture content 
of each cleaned and scoured sample was made previous to tempering. 
The tempering period ranged from 18 to 22 hours. The amount of 
temper water added depended upon the moisture content of the 
individual samples, enough being added to raise the moisture content 
to 15 per cent. During tempering, the samples were kept in air-tight 
cans which prevented any loss of moisture from the wheat from the 
beginning of the tempering period until the wheat started through 


BULLETIN 1013, U. S. DEPARTMENT OF AGRICULTURE. 


the first break rolls. In making the tests, care was taken to collect 
all the material ground. That some weight was lost through the 


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dusting out of fine particles into the air is quite probable, but the 
principal difference in weight between the wheat plus the temper 
water and the resultant mill products was due to evaporation during 
the process of milling. 






























































































































































INFLUENCE OF RELATIVE HUMIDITY AND MOISTURE CONTENT. 5 

The percentage yields indicated by the curves shown in the accom¬ 
panying figures are based on the weight of the cleaned and scoured 
wheat samples before the addition of the temper water. 

INFLUENCE OF RELATIVE HUMIDITY ON TOTAL YIELD OF MILL 

PRODUCTS AND INVISIBLE LOSS. 

In figure 2 is shown the average total yield of mill products for 
various ranges of relative humidities. Because of the influence ex- 

RELAT/VE HUMIDITY - PER CENT 



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Fig. 2.—Showing that the invisible loss in milling decreases and the total weight of mill 
products increases with each increase in the relative humidity of the air within 
the mill. 


erted on yields by the variation in the quantity of temper water 
which differences in the original moisture content of the various 
samples made it necessary to add in order to temper each of 15 percent 
moisture, the curve shown represents the combined effect of relative 
humidity and moisture content of wheat oh the total yield of mill 


































6 BULLETIN 1013, U. S. DEPARTMENT OF AGRICULTURE. 

products. The shaded portion marked “ invisible loss ” is the differ¬ 
ence in the weight of the wheat with the temper water added and the 
total weight of mill products obtained from milling. With each 
increase of relative humidity there is an appreciable decrease in the 
invisible loss occurring during milling and a proportionate increase 
in the total yield of mill products. For example, the average in¬ 
visible loss resulting from milling 11 samples at relative humidities 
ranging from 25 to 29 per cent, inclusive, was 5.2 per cent, while in 
milling 59 samples at relative humidities ranging from 65 to 69 per 
cent, the average invisible loss amounted to only 3.3 per cent. This 
loss in weight is due principally to the evaporation of moisture con¬ 
tained in the wheat, and, except in the cases of low atmospheric 
humidity, amounted to less than the quantity of water added in tem¬ 
pering. In other words, the weight lost through the evaporation of 
moisture during milling was greater than the weight of the temper 
water added when the samples were milled at relative humidities 
lower than 35 to 39 per cent and less when milled at higher relative 
humidities. 

COMPARISON OF THE INFLUENCE OF RELATIVE HUMIDITY AND 
OF MOISTURE CONTENT OF WHEAT ON MILLING YIELDS. 

That both the relative humidity of the air and the moisture content 
of the untempered wheat have a considerable influence on the weight 
lost or gained during milling as compared with the w T eight of the 
wheat before tempering is quite evident from figure 3. In preparing 
this diagram the samples were grouped according to certain ranges 
in the percentages of moisture contained in the wheat samples before 
tempering, and a separate curve was made for each group. This 
scheme of diagramming makes it possible to show the relation of 
both relative humidity and moisture content to the total yield of 
mill products. 

The curves show that the higher yields are associated with the 
higher relative humidities and with the wheats of lower moisture 
content. Each 10 per cent increase in relative humidity above 35 
per cent shows an average increase of about one-half of 1 per cent in 
the total weight of the products obtained, and each decrease of 1 per 
cent in the moisture content of the wheat before tempering shows 
about six-tenths of 1 per cent increase in yield. Compared with the 
weight of the wheat before tempering, the samples containing from 
9 to 9.9 per cent moisture showed a net gain in weight of total mill 
products for all relative humidities given, while the samples contain¬ 
ing from 12 to 12.9 per cent moisture before tempering showed a net 
loss in weight of total mill products except for relative humidities 
higher than 60 to 69 per cent. The greatest average gain in weight 


INFLUENCE OF RELATIVE HUMIDITY AND MOISTURE CONTENT. 7 


for any group was 3.8 per cent and the greatest average loss was 
1.4 per cent. 

In connection with the conclusions expressed in the preceding 
paragraph, it should be borne in mind that the increase in the total 
yield of mill products associated with the higher relative humidities 
was due, no doubt, to decreased evaporation during milling of the 

RELATIVE HUMIDITY - PER CENT 


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Fj G< 3._Relation of the relative humidity of the air within the mill and the moisture 

content of wheat to the total weight of mill products. 


moisture contained in the wheat and in some cases to the absorption 
of moisture from the air by certain mill stocks, and, furthermore, 
that the increase in yields associated with the lower moisture content 
wheats was due to the greater amounts of water which it was neces¬ 
sary to add in tempering in order to bring the final moisture content 
of these wheats up to 15 per cent. The amounts of temper water 



























8 


BULLETIN 1013, U. S. DEPARTMENT OF AGRICULTURE. 


added to the samples, although not definitely stated in the diagrams, 
may be ascertained by subtracting from 15 per cent the percentage 
of moisture contained in the wheat before tempering. 

INFLUENCE OF RELATIVE HUMIDITY ON MOISTURE CONTENT 

OF FLOUR. 

Concerning the influence of atmospheric humidity on the moisture 
content of flour, Bailey 3 in his investigations on that subject con¬ 
cludes that “ flour responds readily to changes in the humidity of 

RELATIVE HUMIDITY - PER CENT 


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Number of Tests 

Fig. 4 .—Showing that as the relative humidity of the atmosphere within the mill in¬ 
creases there is a corresponding increase In the moisture differential of the flour and 
of the wheat from which it is milled. 

surrounding air, the rate at which equilibrium in moisture is ap¬ 
proached depending apparently upon conditions of exposure.” 

An indication of the effect of relative humidity on the moisture 
content of the flour produced in connection with the experimental 
milling tests herein described is shown in figure 4. The results in¬ 
dicate a decided tendency for the moisture content of the flour 
product to be highest for the higher relative humidities. At 
relative humidities ranging from 35 to 39 per cent, the average 

3 Bailey, C. H. The Hygroscopic Moisture of Flour Exposed to Atmospheres of Differ¬ 
ent Relative Humidity, Journal of Industi'ia! and Engineering Chemistry, vol. 12, No. 11, 
p. 1102. Nov., 1920. 


























INFLUENCE OF RELATIVE HUMIDITY AND MOISTURE CONTENT. 9 

* 

moisture content of the flour produced was 12 per cent, and at 65 to 
69 per cent relative humidity, the moisture content was 13.3 per cent, 
or an increase of 1.3 per cent moisture for a difference of 30 per cent 
in relative humidity. 

It has been suggested that probably these are the normal moisture 
contents for flour at the respective relative humidities shown, but 
such a conclusion is not entirely justified, for the reason that in these 
tests no proof exists that the exposure of the flour to the air, which 
was limited to the length of time necessary for milling the sample, 
was sufficiently long to establish a state of equilibrium between the 
moisture content of the flour and that of the mill atmosphere. On 
the other hand, proof that at least some of these moisture contents 
are not normal is evidenced by the wide divergence from the results 
obtained by Bailey, to which reference has been previously made. 
For instance, he found that the hygroscopic moisture in flour in 
equilibrium with atmospheric humidity at 25° C. ranges from a little 
more than 5 per cent at 30 per cent relative humidity to 15 per cent 
at 60 per cent relative humidity. 

COMPARISON OF THE INFLUENCE OF RELATIVE HUMIDITY AND 

OF MOISTURE CONTENT OF WHEAT ON THE MOISTURE CON¬ 
TENT OF FLOUR. 

Some indication of the comparative influence exerted on the mois¬ 
ture content of flour by each of the factors, atmospheric humidity and 
moisture content of the wheat before tempering, is given in figure 5. 
In this figure the results from samples which before tempering had 
moisture contents falling within various ranges are represented by 
separate curves. From this system of grouping it is possible to ascer¬ 
tain the particular influence exerted by each of these two factors. 
The proximity of the four curves to each other and their tendency 
to cross and recross each other indicate, for the method of tempering 
used in these experiments, no very pronounced relation between the 
moisture content of the flour and the moisture content of the wheat 
before tempering; or, in other words, no relation to the quantity of 
temper water added. On the other hand, a decided tendency is shown 
for the moisture content to increase as the relative humidity increases, 
a difference of 10 per cent in relative humidity making an average 
difference of approximately one-half of 1 per cent in the moisture 
content of the flour. 

It would be interesting to know what influence, if any, variations 
in the final moisture content to which wheat may be tempered and the 
length of the tempering period would have on the resultant moisture 
content of the flour milled at different relative humidities. Since all 
samples were tempered to the same moisture content and for approxi¬ 
mately the same length of time, it is impossible to draw any conclu¬ 
sions in regard to this influence from the results of these experiments. 


10 BULLETIN 1013, U. S. DEPARTMENT OF AGRICULTURE. 


Judging, however, from results obtained from other experiments, 
both the extent to which the moisture content of the wheat is raised 
in tempering and the length of time of tempering have an influence 
on the resultant moisture content of the flour product. 


INFLUENCE OF ATMOSPHERIC TEMPERATURE ON THE TOTAL 

YIELD OF MILL PRODUCTS. 

In addition to the influences of atmospheric humidity and moisture 
content of wheat, the possible influence of the temperature of the 
air within the mill on the total yield of mill products has also been 


RELATIVE HUMIDITY - PER CENT 





15 62 

NUMBER OF 


31 


Tests 


Fig. 5.—Relation of the relative humidity of the air within the mill and of the moistuie 
content of wheat to the moisture content of the resultant flour. 


given consideration and the results are represented by the curve 
shown in figure 6. In considering this factor, only the results ob¬ 
tained from those samples having approximately the same moisture 
content (10 to 10.9 per cent) before tempering, and which were 

milled at approximately the same relative humidity (50 to 59 per 
cent) were taken, in order that the results shown might be free from 
the influences of atmospheric humidity and moisture content. The 
curve in figure 6 shows the total yield of mill products obtained for 
various ranges of air temperature, and judging from the indef¬ 
inite direction taken by this curve, no apparent relationship 
















INFLUENCE OF RELATIVE HUMIDITY AND MOISTURE CONTENT. 11 


between air temperature and yield of products can be established 
from the results of these investigations. 

It is not contended that the experimental milling results given in 
this bulletin are entirely indicative of those occurring in commercial 
milling, but it is reasonable to believe that relative humidity and 

AIR TEMPERATURE OF MILL - DEGREES F. 


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Number of Tests 

jr IG 6._Relation of the temperature of the air within the mill to the total weight of 

mill products. 


moisture content of wheat have a similar influence on the pioducts 
of commercial milling plants, so that if any difference in influence 
does occur, it is only in degree and not in kind. 

SUMMARY. 

In brief, the conclusions that may be drawn from these experi¬ 
mental milling tests on hard wheats of various moisture contents, all 



















12 


BULLETIN 1013, U. S. DEPARTMENT OF AGRIC 


0 018 625 31 


tempered to the same final moisture content of 15 per cent, are as 
follows: 

With each increase of relative humidity or, in other words, with 
each increase in the extent to which the air within the mill became 
saturated with moisture, there was an appreciable decrease in the in¬ 
visible loss occurring during milling and a proportionate increase 
in the total yield of mill products. 

The higher total yields of mill products were associated with the 
higher relative humidities and with the wheats of lower moisture 
content. These higher yields in the former case were undoubtedly 
due to decreased evaporation during milling of the moisture con¬ 
tained in the wheat and probably to the absorption of some moisture 
from the atmosphere by some of the mill stocks; and, in the latter 
case of lower moisture content wheats, to the greater amounts of 
water which it was necessary to add in tempering these wheats in 
order to bring their moisture contents to the required percentage. 
Each 10 per cent increase in relative humidity above 35 per cent 
increased the total weight of the products obtained about one-half 
of 1 per cent, and each decrease of 1 per cent in the moisture content 
of the wheat before tempering caused about six-tenths of 1 per cent 
increase in yield. Compared with the weight of the wheat before 
tempering, the samples containing from 9 to 9.9 per cent moisture 
showed a net gain in weight of total mill products for all relative 
humidities given, while the samples containing from 12 to 12.9 per 
cent moisture before tempering showed a net loss in weight of total 
mill products except for relative humidities higher than 60 to 69 
per cent. 

Xo very pronounced relation between the moisture content of the 
flour and that of the wheat before tempering was shown, or in 
other words, no relation was shown of moisture content of flour to 
the quantity of temper water added to the wheat. On the other hand, 
a decided tendency was shown for the moisture content of the flour 

•y 

to increase as the relative humidity increased, a difference of 10 per 
cent in relative humidity, making an average difference of approxi¬ 
mately one-half of 1 per cent in the moisture content of the flour. 

Xo definite relation between air temperature and total yield of 
mill products was established. 


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GOVERNMENT PRINTING OFFICE 
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V 


WASHINGTON : GOVERNMENT PRINTING CFFICE : 1921 





